The invention pertains to a carrier matrix for catalysts with cross-flow effect, as well as enlarged surfaces and improved retaining capacity as compared to catalysis-promoting metal oxides present in dipping dispersions, comprising superposed layers of high temperature and scale resistant steel; such a carrier matrix coated with metal oxide; and a process for the production of the last mentioned matrix. Another object of the invention is the use of these matrixes for the production of catalysts.
The pollutants, especially of exhaust gases from the internal combustion engines of the continuously increasing number of motor vehicles, represent a considerable danger for the health of humans, animals and plants. They are already delimited in a few countries by legal determination of maximally permissible concentrations of pollutants. Among the methods proposed and already practiced for the solution of these air pollution problems, the exhaust gas purification processes have become most important. The greatest demands are made of the catalysts required for this purpose regarding heating-up behavior, effectiveness, endurance activity and mechanical stability. For example, in case of use in motor vehicles, they must become active at the lowest possible temperatures and they must guarantee operation, for a long time and in all pertinent ranges of temperature and volume rates to obtain a high percentage conversion of the pollutants to be converted (especially hydrocarbons, carbon monoxide and nitric oxides) into harmless oxidation and reduction products, such as carbon dioxide, steam and nitrogen. Because of the great mechanical strains during continuous operation they must have a sufficient mechanical stability and must not lose it even in the event of extended overheating as might possibly occur as a result of being acted upon by unburned fuel, for example in the event of ignition failure in one or more cylinders. Therefore, they must in this case satisfy a series of conditions which are difficult to fulfill simultaneously or are contradictory to one another.
Hitherto, monolithic carrier catalysts have been used above all next to bulk-bed catalysts, i.e., mouldings or extrudates of carrier catalysts or interspersant or mixed catalysts. They consist of an inert, low-surface, ceramic skeleton of, for example, cordierite, mullite or .alpha.-aluminum oxide as structural reinforcer, on which a thin, mostly high surface layer of heat resistant, mostly oxidic carrier material, such as aluminum oxide of the so-called gamma-series is applied, which latter in turn carries the actual catalytically active component.
This may be of noble metal, noble metal compounds or non-noble metal compounds. From the group of the noble metals, for example, platinum, palladium, rhodium, ruthenium, iridium, gold and silver may be used. As non noble metal compounds for example, the oxides of copper, chrome, manganese, iron, cobalt, nickel and their combinations as for example, copper chromite are contemplated. Other variations are included developed by combining noble metals or their compounds with non-noble metals or their compounds, or else non-noble metals or their compounds with noble metals or their compounds. In many cases, small quantities of other elements are still added to the active component, such as from the group of the alkaline-earth metals, e.g., magnesium calcium, strontium or barium, from the group of the rate earths, e.g. samarium, lanthanum, cerium, or from the IV group of the periodic system, e.g. titanium, zirconium or tin, as so-called promoters for the improvement for certain characteristics of the system.
The poor heat conductivity and sensitivity to mechanical shock and thermal overheating turned out to be a considerable disadvantage of the catalysts with ceramic structural reinforcers, especially the monolithic honeycomb catalysts made from cordierite, mullit or .alpha.-aluminum oxide. Thus the shocks occurring in travel as a result of intermittent impulses of the exhaust gas column, the vibrations of the motor and movements of travel in connection with peak temperatures have a shattering and crumbling effect on the ceramics. In the event of a thermal overheating taking place in the narrow confines of space of the catalysts, a sintering, melting and baking of the structural reinforcer being present, in the form of monoliths or bulk bodies, with its surface coatings, may occur resulting in partial or complete deactivation.
It has turned out furthermore that the attachment of such ceramic honeycombs in metal housings is difficult because of the variation in heat expansion of ceramics and metal, and requires expensive preparations in construction, in order to guarantee an elastic and gastight support of the honeycombs at given the operating temperatures the latter changing continually within the possible scope of -30.degree. and +1000.degree. C.
Therefore, all sorts of efforts were made, for finding better suitable replacement materials for the catalysts based on a ceramic base and in searching for a more favorable spacial configuration for them.
Thus, as early as in the German OS No. 23 02 274 a carrier matrix for a catalyst reactor for the purification of exhaust gas in internal combustion engines had been described, which is made of alternately disposed corrugated and smooth high temperature resistant sheet steel which is coated with a catalytically active metal, such as platinum or palladium or a metal oxide such as copper oxide, nickel-oxide or something similar. Herein it is said to be also known to use metal carriers from materials with a high nickel content (Monel), whereby the nickel, after conversion into oxide shows catalytic activity. The above-mentioned German OS proposes, among other things, to coat the sheets of steel with copper or nickel and to oxidize the coating subsequently or to cover it directly with a catalytically active metal oxide.
In addition, the German OS No. 24 40 664 describes a catalyst built up of a maximum of four layers, wherein a carrier capable of resisting heat and oxidation, made of an iron alloy in the form of an extended sheet wound up in a roll, is coated with a porous coating, containing oxygen, which is to carry the catalyst layer disposed thereon, and is obtained preferably by thermal, chemical or electrolytic surface oxidation of an iron alloy, containing aluminum, with possible subsequent reinforcement by means of aluminum oxide, applied from outside.
Disadvantages of such metallic structural reinforcers for catalyst materials are to be seen in the fact that they have a limited geometrical surface, which limits the retaining capacity as compared to catalysts promoting, high surface, heat resistant metal oxides such as .gamma.-Al.sub.2 O.sub.3 made with dipping dispersions, to the point that one requires several repetitions of the dipping process for achieving a coating of said oxides, sufficiently strong for impregnation with the actual catalytically active component. Since the known carrier matrixes are traversed by separated flow channels, the gas mixture that is to be converted comes into contact with the catalyst material only in the form of longitudinally flowing gas columns enclosed by the walls of the flow channels; as a result of that, in the case of a predetermined gas-flow velocity, a certain often overly great minimum length of the matrix is required, in order to achieve a satisfactory mass transfer and tied to that a sufficient degree of conversion. Finally heat gradients occur between the individual longitudinally running, discrete reaction zones, for example on the basis of locally more or less variable thickness of layers and activities of the catalyst material, which can be balanced out only by way of the specific heat conductivity of the wall material of the channel.